1. Field of the Invention
The invention concerns polyamide molding compositions and a method for the preparation thereof with greatly improved processability, flow paths and surface quality, without loss of strength or thermal stability, a method for the preparation thereof and use thereof, for the manufacture of molded bodies such as fibers, sheets, pipes, hollow bodies or other half-finished or products e.g. by extrusion, pultrusion, injection molding, injection blow-molding or other shaping techniques.
The invention is characterized in that quantities of up to 20 wt. % of the polyamide matrix of prepolymers of the same or different polyamides are incorporated into the polyamide molding compositions, resulting in a great improvement in flowability and mold removal behavior as well as surface quality without strength and thermal stability being reduced. The polyamide molding compositions according to the invention mainly concern those which are provided with reinforcing agents of all kinds or certain additives, but also molding compositions which contain no additives. The invention further concerns the use of the molding composition.
2. Description of the Prior Art
Additives are known for improving the processability of polyamide molding compositions for extrusion or injection molding. Thus amides or urea derivatives are proposed as an additive for improving the flow properties as well as given amides, diamides or imides.
For this purpose the addition of silicone oils, stearic acid or calcium or zinc salts thereof or long-chain aliphatic esters and alcohols is described as well.
But these additives often have the drawback that they impair the quality of the polyamide molding compositions by the fact that they have a tendency to exudation i.e. migration or also smoke formation during processing or are extracted out by solvents or form streaks and spots or cause a reduction of molecular weight and so lower strength, rigidity and thermal dimensional stability and the like (Nylon Plastics Handbook, Melvin I. Kohan, pages 436–440).
Also it is known that various additives can be incorporated to improve the surface gloss. Here the use of polyhexamethylene adipic acid amide or polyhexamethylene isophthalic acid amide with a viscosity of 1.5–2.8 (1% in sulphuric acid) in combination with copper, iodine or manganese salts or the addition of polyphenyl ether may be mentioned. On the other hand resin materials from polyamides with good surface gloss with a greatly increased proportion of inorganic fillers were also developed. To increase strength and rigidity the use of terephthalic acid and isophthalic acid in polyamide resins is proposed. However it is difficult to make molded bodies with satisfactory surface quality from them, because they exhibit poor flowability.
Likewise the improvement of thermal stability of molding compositions is known. In DE 4 311 885 additions of alkali salts of oxalic, malonic, glutaric or phthalic acid are recommended for this purpose.
But here it is necessary, for the simultaneous improvement of several important properties of the polyamide molding compositions such as flow behavior, gloss, strength and thermal stability, to use additives of different kinds. But these additives on the one hand are not always compatible with each other, and on the other hand they often impair important additional properties such as the toughness, the flexibility, the braking elongation, the stress cracking resistance, etc.
In EP 0 272 695 are described high-impact-strength, multiphase, thermoplastic polyamide mixtures which are provided with 10–40 wt. % of impact strength modifiers of the type EP, EPDM, SEBS, SBS, SAN and the like and, in addition to the high-molecular polyamide, also contain a proportion of lower molecular polyamide units to improve processing. The lower-molecular polyamide has the function of lowering the higher viscosity caused by impact strength modification again and so making the molding compositions more processable; it has a high unbalanced terminal group content, a mean molecular weight of <5,000 and an inherent viscosity of <0.5 dl/g, so that the polyamide mixture finally achieves a resulting viscosity of 5–50 Pa.s (50–500 poise) or a fusion of 100→200 g/10 minutes at 280° C. and 2160 g load according to ASTM D 1238.
The method described is elaborate as it requires two extrusion steps and is unsatisfactory insofar as the polyamide moldings manufactured often turn out stained particularly at higher concentrations of the foreign-polymer impact strength modifiers and exhibit streaks in the case of larger moldings, moreover the low-molecular polyamide reacts unevenly with the high-molecular polyamide due to partial postcondensation and transamidation, as a result of which great fluctuations in viscosity can arise.
An additional feature of the EP 0 272 695 reference is the inclusion of 10–40% of an impact strength modifier, and additionally, a lower molecular polyamide to improve processing. The present invention, does not need these impact strength modifiers as does the EP 0 272 695 reference, and in fact, can achieve superior properties (e.g. tensile strength, elongation and E-modulus) than does the EP 0 272 695 reference. These superior results are identified below, in tables 1–3 reflecting the present invention, and table 4, a comparative example reflective of the Epstein reference.
EP 0 553 617 discloses polyamide molding compositions with a high content of fillers in which the fillers are mixed in the melt with a low-molecular polyamide with a viscosity number of 40–80 ml/g (measured at 25° C. in 0.5% H2S04 96%) and are brought to higher viscosities in the solid phase by a separate postcondensation step. The lower-molecular polyamide is intended to have a terminal group content which is equimolar as far as possible and not too high. This method has the drawback that two work stages are necessary and high fluctuations in final viscosity must be expected, as a result of which moldings often turn out rough, stained and streaky, and postcondensation is time-consuming and gives rise to defective processability of the molding compositions.